1. Field of the Invention
The present invention relates to a wavelength conversion member that contains phosphors made of phosphor particles each coated with a coating, and a light-emitting device having the phosphors and a semiconductor light-emitting element such as an LED or a semiconductor laser combined therein.
2. Description of the Background Art
A light-emitting device that converts light emitted from a semiconductor light-emitting element such as a light-emitting diode (LED), by means of phosphors, has a small size, and less power consumption than a light bulb. Accordingly, the light-emitting device has been put into practical use to serve as a light source for various types of display devices or illuminating devices. The light-emitting device is under development to achieve high efficiency or high reliability.
Japanese Patent Laying-Open No. 2002-171000 discloses a light-emitting device that emits white light by using a semiconductor light-emitting element emitting light having a wavelength of 390 nm to 420 nm, and phosphors excited by the light emitted from the semiconductor light-emitting element. As the phosphors made to emit light by the excitation light having a wavelength of 390 nm to 420 nm, there are used a variety of phosphors made of an oxide or a sulfide.
Unfortunately, however, some phosphors such as phosphors containing a sulfide may be reacted with moisture in the air and hydrolyzed. Such degradation of the phosphors would shorten a useful life of the light-emitting device. To deal with the problem, Japanese Patent Laying-Open No. 2002-223008 discloses phosphors each having a coating.
In recent years, instead of the oxide phosphors or sulfide phosphors, an example of oxynitride phosphors or nitride phosphors are disclosed in Japanese Patent Laying-Open No. 2002-363554 and Japanese Patent Laying-Open No. 2003-206481. Many of these phosphors have excellent properties such as capability of emitting light highly efficiently by being excited by the light having a wavelength of 390 nm to 420 nm, high stability, high water resistance, and less fluctuation in luminous efficacy, which fluctuations are caused by variations in operating temperature.
In order to further improve heat resistance of the nitride phosphors and others, Japanese Patent Laying-Open No. 2004-161807 discloses provision of a coating made of a metal nitride-based or metal oxynitride-based material. According to this document, when (Sra, Ca1−a)xSiyOzN{(2/3)x+(4/3)y−(2/3)z} : Eu, x=2, y=5 is produced as the oxynitride phosphors, it is prone to baking degradation, and hence each of the phosphors is coated with a coating containing an N element. As the coating containing an N element, there are used a metal nitride-based material containing nitrogen and a metal such as aluminum, silicon, titanium, boron, or zirconium, and organic resin containing an N element, such as polyurethane or polyurea. According to this document, in nitrogen phosphors not provided with the coating containing an N element, luminous efficacy is drastically decreased when the phosphors are heated to 200-300° C., whereas in nitrogen phosphors provided with the coating containing an N element, decomposition of nitrogen is reduced owing to a supply of nitrogen, and hence heat resistance is improved.
Furthermore, Japanese Patent Laying-Open No. 2004-071357 discloses a light-emitting device in which phosphors are arranged such that a light-emitting element, red phosphors, green phosphors, and blue phosphors are arranged in this order, so that reabsorption of light emitted from the phosphors located on a side near the light-emitting element is suppressed.